Automated welding of moulds and stamping tools

ABSTRACT

A tool welding system is disclosed that includes a table that heats a tool. A multi-axis robot includes a welding head that is moved relative to the table in response to a command. A controller is in communication with the robot and generates the command in response to welding parameters. The weld parameters are based upon a difference between an initial tool shape and a desired tool shape. The difference between the initial tool shape and the desired tool shape corresponds to a desired weld shape. The desired weld shape is adjusted based upon initial tool shape variations, which includes thermal growth of the tool. The tool is welded to provide the desired weld shape to achieve a desired tool shape.

BACKGROUND

This disclosure relates to a method and system for welding tools such asthose used for moulding and stamping. More particularly, the disclosurerelates to a method and system for welding additional material onto atool to be reworked, for example, for subsequent use in producingproducts in need of a class A surface.

Tools, such as stamping tools and plastic injection moulds, must bewelded for a variety of reasons. During the repair of tools for cracksor wear, it is often necessary to grind out material and then build upthe ground surface to provide new material. The newly welded material isthen partially machined away to create a new surface that matches therequired design surface.

Additionally, there are occasionally part changes that deviate from theinitial part design. Part changes require a corresponding change in thetool. If this change involves only the removal of material from thetool, then material can be simply machined away. If however the partdesign change requires addition of material to the surface of the tool,as it typically does, then additional material must be added to thedesired area. This is accomplished through the application of successivelayers of weld material until the required thickness of material isadded prior to machining. The required thickness of material may be ashigh as 2 inches (50 mm) requiring numerous layers of weld material tobe applied.

Because of the high surface quality required for many tool surfaces(particularly those being built to Class A automotive standards), andthe additional risk of distortion of the welded surface, the tool mustbe welded using tungsten inert gas (TIG) welding at an elevatedtemperature of approximately 700° F. (370° C.). When such welding iscarried out using manual techniques, the welder must be provided withprotective gear and suitable cooling when working in this very harshenvironment. Often the tool can only be heated to approximately 400° F.(210° C.), which is less than desired, to accommodate the welder.

Robotic welding has been experimented with in various fields ofindustry. For example, robotic welding systems for rapid prototypinghave been suggested. Such systems have been very conceptual in natureand do not lend themselves to the unique environment and challenges ofwelding tools that require class A surfaces. These large tools,typically weighing several tons, thermally expand as much as a half aninch (12 mm) or more as they are heated.

A typical application in tool modification is to build up a rectangular,circular, triangular, or arbitrarily shaped area on the surface on thetool. This is accomplished by laying down parallel passes of weld metalon the area to be built up and then repeating this process to build upmultiple layers, one at a time, until the required metal thickness isachieved. This is a very time consuming process and requires theinvestment of substantial man hours of welding in order to achieve therequired surface shape. A highly skilled tool welder can typically onlyweld about a half a pound of material per hour. The boundary of themanually welded area typically varies such that a more than desiredamount of welded material must be removed during final machining. Thisis because a typical welder cannot achieve and maintain the contour ofthe outer boundary throughout the welding process. Manual welderssometimes weld a perimeter as a guide so that they more accurately laydown the desired weld shape to the area.

What is needed is an automated welding method and system that issuitable for tool welding in heated environments.

SUMMARY

A tool welding system is disclosed that includes a table having burnersin communication with a fuel source. The table heats a tool to a desiredtemperature, which enables better surface matching needed to produce aclass A surface. A multi-axis robot includes a welding head that ismoved relative to the table in response to a command. A controller is incommunication with the robot and generates the command in response towelding parameters. The weld parameters are based upon a differencebetween an initial tool shape and a desired tool shape. The tool isprobed in some fashion, in one example, to correlate the initial toolshape data to the tool's position on the table. The difference betweenthe initial tool shape and the desired tool shape corresponds to adesired weld shape that represents the material that will be welded ontothe tool. The desired weld shape, which consists of multiple passes orlayers, is adjusted based upon initial tool shape variations, whichincludes thermal growth of the tool. The tool is welded to provide thedesired weld shape to achieve the desired tool shape. In one example,the perimeter of the desired weld shape in each pass is welded first andthen filled in by additional adjoining weld beads. Adjustments are madethroughout the welding process based upon variations in the weld beadsand tool.

These and other features of the application can be best understood fromthe following specification and drawings, the following of which is abrief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly schematic view of an example tool welding system.

FIG. 2 is a flow chart depicting an example embodiment of a method ofwelding a tool.

FIG. 3 is a schematic view of one pass or layer of a desired weld shapeapplied to an initial tool shape to achieve a desired tool shape.

FIG. 4 is a schematic view of a welding head and feed system in theprocess of welding the tool.

FIG. 5 is a enlarged view of a portion of a perimeter weld bead and anadjoining weld bead.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A welding system 10 is schematically shown in FIG. 1. The system 10includes a table 12 that supports a tool or mould 14 that is to bereworked by adding welded material to its surfaces. In one example, thetable 12 is connected to a fuel source 16 that provides fuel to burnersin the table 12 to heat the mould 14 to temperatures of at leastapproximately 400° F., in one example, and as much as 700° F. or more inanother example. A heater control system 18 is associated with the table12 and the fuel source 16 to regulate the heat provided by the table 12with valves and additional hardware and/or software. The mould 14 canalso be heated using an electric heat source, for example.

A multi-axis robot 20 is arranged atop a pedestal 22 near the table 12.The robot 20 includes a base 25 mounted to the pedestal 22 and arms 24that support a welding head 26. In one example, the welding head 26 is agas tungsten arc welding (GTAW) configuration having a electrode 28. Awire feeder 30, which is schematically shown in FIG. 1, continuouslysupplies wire to the electrode 28, which is transformed into plasma inresponse to a current from a power source 35. A shielding gas 29 isconnected to the welding head 26 to shield the welding site, as isknown.

In one example, the robot 20 is a five axis device. The arms 24 rotatetogether relative to the base 25 about an axis X. The arms 24 pivotabout independent axes Y and Z. The welding head 26 is rotatable aboutan axis A and pivots relative to an end of one of the arms 24 about anaxis B. In this manner, the position of the electrode 28 can bemanipulated in a highly accurate manner. In the example shown, the robot20 is positioned above the table 12 to minimize any positionaltranslation errors that are more likely to occur with the welding headnear the extremity of its reach.

The accuracy of the robot 20 can be adversely affected by heat from thetable 12 and mould 14. Accordingly, it may be desirable to providecooling passages 32 within various components of the robot 20 that arein communication with a cooling system 34. Cooling the robot 20 preventsthe temperature of the sensitive components of the robot from exceedinga predetermined temperature, or to minimize thermal growth of the robot.

The system 10 also includes a controller 38 that is in communicationwith a variety of components for calculating and achieving a desiredweld shape to produce a desired tool shape corresponding to the reworkedinitial tool shape. The controller 38 may include hardware and softwarethat can be integrated or separated into modules. The controller 38 isin communication with a CAD database 36. The CAD database 36 mayinclude, for example, three dimensional data that provides the initialtool shape of the mould 14 in need of rework. The CAD database 36 mayalso include a desired tool shape, which corresponds to the desiredshape of the reworked mould. The controller 38 is programmed to comparethe data relating to the initial and desired tool shapes to determine adesired weld shape. The controller 38 interrelates the robot coordinatesystem and the mould dimensional information, which allows thecoordinates of the CAD database 36 to be used in generating the robotwelding paths. The desired weld shape represents the weld that will belaid down on the mould 14 to rework it. The desired weld shape willtypically be broken into multiple welding passes or layers that are laidon top of one another.

The controller 38 determines weld parameters 40 based upon a comparisonof the initial and desired tool shapes. Some of the welding parameters40 may be determined by one or more manually input values from a systemoperator. For example, the operator may input desired pounds of weldedmaterial per hour. The welding parameters 40 include, for example,electrode trajectory 46, welding speed 48, current 50, electrodeorientation 52 and wire feed rate 54. Parameters such as welding speed48 and current 50 can be determined empirically for a known “good” weldat a given pound per hour welding rate. This known information is thenused to determine the other parameters. During the welding process it ispossible to use the magnitude of the current flow in the weld arc tosense distance between the tip of the tungsten electrode and the surfacebeing welded. This current flow magnitude can then be used in a closedloop control system to adjust the robot position above the surface on acontinuous basis, providing a superior quality of weld.

A feedback system 42 is in communication with the controller 38. Thefeedback system 42 includes, for example, a voltage sensor 56, a forcesensor 58, a wire sensor 60 and other sensors 61. The coordinates 36relating to the initial tool shape can be interrelated to the tool'sposition on the table 14 using an optically based system using camerasand photogrammetry techniques, or can be based on mechanical probing ofthe tool 14 using the robot movements and a touch trigger probe.

In one example, the voltage sensor 56 is used to maintain a desireddistance between the electrode 28 and the mould 14 such that itcorresponds to a desired voltage for a good weld. The force sensor 58may be provided in one or more joints or locations of the robot 20 andare tripped in the event of a collision between a portion of the robot20 and the mould 14. The sensitivity of the force sensor 58 can bechanged throughout the welding process depending upon, for example, theelectrode position.

Turning to FIG. 4, the wire feeder 30 is shown in more detail. The wirefeeder 30 includes a spool 100 that feeds wire 102 to electrode 28 inthe welding head 26. The spool 100 includes a feature 103, in oneexample, that cooperates with the wire feed rate sensor 54, such as aproximity sensor, to detect the rate at which the wire 102 is fed to theelectrode 28. The wire sensor 60 detects the presence of the wire 102.In the event that the spool 100 runs out of wire, a signal is sent tothe controller 38 to generate an error. If the welding head 26 isstationary while the spool 100 continues to feed wire 102, as detectedby the wire feed rate sensor 54, an error will be generated.

With continuing reference to FIG. 4, the wire feeder 30 includeslocating features for locating the wire 102 relative to the electrode 28precisely subsequent to servicing the welding head 26. The wire 102 isfeed to the electrode 28 through a feed tube 106 that is received in arecess 116 in the side of the welding head 26. The feed tube 106includes a flange 118 that is located by a slot in the recess 116 toaxially position the feed tube 106. An end 108 of the feed tube 106 isreceived in an aperture 112 in a nozzle 110, which is in communicationwith the shielding gas 29. In this manner, the location of the wire 102relative to the electrode 28 can be quickly and repeatably achieved.

The other sensors 61 may include, for example, an optical sensor todetermine the position of the electrode 28 relative to the mould 14 andmake adjustments to accommodate for a worn mould or other initial toolshape variations such as thermal growth of the tool from the heatedtable 12. The temperature sensor 44 may also provide feedback to thecontroller 38 to account for thermal growth of the mould 14 orcomponents of the robot 20 to make adjustments to the desired weld shapeor electrode position. Thermal growth of the mould 14 at 700° F. can beas much as a half an inch (12 mm) or more, which significantly impactsthe welding path and welding parameters needed to achieve the desiredweld shape at the desired location on the mould 14.

Referring to FIGS. 3 and 4, the mould 14 includes a first surface 62adjacent to second and third surfaces 64, 66. In the example shown, thefirst surface 62 corresponds to a generally first wall, and the secondsurface 64 provides an inclined wall 68 relative to the first surface62. The third surface 66 forms an edge 70 relative to the first surface62. The desired weld shape is divided into first and second passes 74,76 that correspond to generally parallel welding planes. The weldingparameters 40 are typically chosen to maximize straight line weldingspeed. A manual welder will lay down adjacent beads in the samedirection since it is easier to work from one side of the welder's body.In one example, the system 10 alternates the direction of adjacent weldbeads, since it is faster to do so.

The welding parameters 40 are adjusted when a weld bead reaches aninclined wall 68 or an edge 70 to achieve the desired penetration andweld bead shape. For example, the orientation of the welding head 26 maybe changed to provide clearance relative to the inclined wall 68.Moreover, it may be desirable to change the orientation of the wirerelative to the direction of the weld bead. It may be desirable toincrease the current when approaching an inclined wall 68 to ensuredesired penetration, while it may be desirable to decrease the currentwhen approaching an edge 70 to prevent roll-off of the weld puddle downthe edge or to prevent over penetration.

The second pass 76 is welded on top of the first pass 74. Due tovariation in height of the weld bead, the position of each pass isadjusted relative to the previous pass. The voltage between theelectrode 28 and mould 14 is monitored by the voltage sensor 56 tomaintain a desired distance of the electrode 28 relative to the surfaceto be welded. The controller 38 may make adjustments to the weldparameters such as reducing or adding passes from the number of passesinitially calculated to achieve the desired weld shape.

Referring to FIG. 3, it may be desirable to weld a perimeter 72 on thefirst surface 62 corresponding to a boundary of the desired weld shape.The perimeter 72 is filled in by multiple, slightly overlapping beadpaths 78, 80 to provide a pass of solid material without any voids.However, the perimeter 72 is not welded for the reasons that it isemployed in manual welding. The robot 20 has no issues with preciselywelding a desired weld shape since it is computer controlled. Instead,it is desirable to first weld a perimeter 72 in the pass to preventroll-off 83 when making a turn 82 between first and second bead paths78, 80, which is shown in FIG. 5. Typically, there is too much weld whenmaking the turn 82 such that the weld bead rolls off or flattensundesirably. This will result in an insufficient amount of material atthe perimeter of the welded area. Roll-off is not an issue with askilled manual welder. However, the adjustments needed to preventroll-off at the boundary of the desired welded shape are difficult toquantify for expression for the robot. When providing a perimeter 72,the roll-off when making the turn 82 between adjoining bead paths 78, 80is contained by the perimeter 72, as best illustrated in FIG. 5.

Referring to FIG. 2, a flow chart of an example welding method 84 isillustrated. The initial mould shape is input to the system 10, asindicated in block 86. The initial mould shape is three dimensionaldigital data, for example. Variations of the mould shape 88 can also bedetermined, as indicated at block 88. An optical sensor or other devicecan be used to determine the position and orientation of the mould 14relative to the robot 14. The variations can correspond to wear to themould if it has already been in use, pre-welding machining, or thermalgrowth of the mould. The desired mould shape is input into the system,as indicated at block 90. The desired mould shape corresponds to areworked mould shape, for example. The weld parameters 40 are determinedand include the speed 48, current 50, electrode orientation 52, wirefeed rate 54 and electrode trajectory 46, for example. The weldparameters can be adjusted to accommodate temperature 44, to account forthermal growth of the mould 14 and/or robot 20. The trajectory 46includes the number of passes 122, bead paths 124 (including directionand number of weld beads), and a perimeter 120 corresponds to a boundaryof the desired weld shape.

Throughout the welding process, the weld parameters 40 can be adjustedto achieve a desired weld bead, as indicated at block 92. The parametersare adjusted based upon voltage from the voltage sensor 56 and forcesfrom a force sensor 58 that can be indicative of an undesired collisionbetween the electrode 28 and the mould 14.

The desired weld shape corresponds to welded material that is withoutany voids and capable of providing a class A surface. The welded mouldis finish machined, as indicated at block 96, to provide a reworkedmould having a class A surface.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

1. A method of welding a tool comprising the steps of: providing a firstset of digital data corresponding to an initial tool shape; providing asecond set of digital data corresponding to a desired tool shape;determining weld path and weld parameters based upon a difference of thefirst and second sets of digital data, which results in a desired weldshape; heating a tool having the initial tool shape; adjusting thedesired weld path based upon initial tool shape variations, theadjusting step including detecting a position of a heated mould surfaceindicative of the thermal growth of the tool; and automating welding ofthe tool according to the weld path and weld parameters to provide thedesired weld shape to achieve the desired tool shape.
 2. The methodaccording to claim 1, wherein the initial tool shape provides a partsurface corresponding to a product, and the desired tool shapecorresponds to a reworked part surface for the same product.
 3. Themethod according to claim 2, wherein the desired weld shape provides thereworked part surface.
 4. The method according to claim 3, comprisingthe step of finish machining the desired tool shape to provide a class Asurface for the reworked part surface.
 5. The method according to claim1, wherein the heating step includes heating the tool above 400° F. 6.The method according to claim 5, wherein the heating step includesheating the tool to approximately 700° F.
 7. The method according toclaim 1, wherein the detecting step includes mechanically probing theheated mould surface.
 8. The method according to claim 1, wherein thedetecting step includes optically sensing the heated mould surface. 9.The method according to claim 1, wherein the detecting step includessensing a temperature associated with the mould.
 10. The methodaccording to claim 1, wherein the welding parameters include at leastone of a welding speed, a current, an electrode orientation, and a wirefeed rate.